Showing papers by "M. B. Maple published in 2018"

Measurements of the NMR Knight shift tensor and nonlinear magnetization in URu2Si2

Abstract: URu$_2$Si$_2$ exhibits an anomalous peak in the nonlinear magnetic susceptibility at the hidden order transition In order to investigate this anomaly, we conducted direct magnetization measurements and investigated the detailed angular dependence of the $^{29}$Si nuclear magnetic resonance Knight shift tensor We find that the nonlinear magnetization is smaller than previously reported, and the analogous nonlinear Knight shift tensor is below the detection limit Our results suggest that the magnitude of the anomalous peak is sample dependent

Polar Kerr Effect from Time-Reversal Symmetry Breaking in the Heavy-Fermion Superconductor PrOs_{4}Sb_{12}.

Abstract: We present polar Kerr effect measurements of the filled skutterudite superconductor ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$. Simultaneous ac susceptibility measurements allow us to observe the superconducting transition under the influence of heating from the optical beam. A nonzero Kerr angle ${\ensuremath{\theta}}_{K}$ develops below the superconducting transition, saturating at $\ensuremath{\sim}300\text{ }\text{ }\mathrm{nrad}$ at low temperatures. This result is repeated across several measurements of multiple samples. By extrapolating the measured ${\ensuremath{\theta}}_{K}(T)$ to zero optical power, we are able to show that the Kerr angle onset temperature in one set of measurements is consistent with the transition to the $B$ phase at ${T}_{C2}$. We discuss the possible explanations for this result and its impact on the understanding of multiphase and inhomogeneous superconductivity in ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$.

Search for multipolar instability in URu 2 Si 2 studied by ultrasonic measurements under pulsed magnetic field

Abstract: The elastic properties of ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ in the high magnetic field region above 40 T, over a wide temperature range from 1.5 to 120 K, were systematically investigated by means of high-frequency ultrasonic measurements. The investigation was performed at high magnetic fields to better investigate the innate bare $5f$-electron properties, since the unidentified electronic thermodynamic phase of unknown origin, the so-called ``hidden order'' (HO), and associated hybridization of conduction and $f$ electrons $(c\text{\ensuremath{-}}f$ hybridization) are suppressed at high magnetic fields. From the three different transverse modes we find contrasting results; both the ${\mathrm{\ensuremath{\Gamma}}}_{4}({B}_{2g})$ and ${\mathrm{\ensuremath{\Gamma}}}_{5}({E}_{g})$ symmetry modes ${C}_{66}$ and ${C}_{44}$ show elastic softening that is enhanced above 30 T, while the characteristic softening of the ${\mathrm{\ensuremath{\Gamma}}}_{3}({B}_{1g})$ symmetry mode $({C}_{11}\ensuremath{-}{C}_{12})/2$ is suppressed in high magnetic fields. These results underscore the presence of a hybridization-driven ${\mathrm{\ensuremath{\Gamma}}}_{3}({B}_{1g})$ lattice instability in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$. However, the results from this work cannot be explained by using existing crystalline electric field schemes applied to the quadrupolar susceptibility in a local $5{f}^{2}$ configuration. Instead, we present an analysis based on a band Jahn-Teller effect.

The effective increase in atomic scale disorder by doping and superconductivity in Ca3Rh4Sn13

Abstract: A comprehensive study of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca3Rh4Sn13 doped with La (Ca3−x La x Rh4Sn13) or Ce (Ca3−x Ce x Rh4Sn13) with superconducting critical temperatures T*c higher than those (T c ) observed in the parent compounds. The T − x diagrams and the entropy S(x) T isotherms document well the relation between the degree of atomic disorder and separation of the high-temperature T*c and T c -bulk phases. In these dirty superconductors, with the mean free path much smaller than the coherence length, the Werthamer–Helfand–Hohenber theoretical model does not fit well the H c2(T) data. We demonstrate that this discrepancy can result from the presence of strong inhomogeneity or from two-band superconductivity in these systems. Both the approaches very well describe the H − T dependencies, but the present results as well as our previous studies give stronger arguments for the scenario based on the presence of nanoscopic inhomogeneity of the superconducting state. A comparative study of La-doped and Ce-doped Ca3Rh4Sn13 showed that in the disordered Ca3−x Ce x Rh4Sn13 alloys the presence of spin-glass effects is the cause of the additional increase of T*c in respect to the critical temperatures of disordered Ca3−x La x Rh4Sn13. We also revisited the nature of structural phase transition at T*~130÷170 K and documented that there might be another precursor transition at higher temperatures. Raman spectroscopy and thermodynamic properties suggest that this structural transition may be associated with a CDW-type instability.

Temperature versus Sm concentration phase diagram and quantum criticality in the correlated electron system Ce 1 − x Sm x CoIn 5

Abstract: We report electrical resistivity, magnetization, and specific heat measurements on the correlated electron system ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Sm}}_{x}{\mathrm{CoIn}}_{5}$ ($0\ensuremath{\le}x\ensuremath{\le}1$). Superconductivity (SC) in the heavy-fermion compound ${\mathrm{CeCoIn}}_{5}$, which is suppressed with increasing Sm concentration $x$, and antiferromagnetic (AFM) order of ${\mathrm{SmCoIn}}_{5}$, which is suppressed with decreasing $x$, converge near a quantum critical point at ${x}_{\mathrm{QCP}}\ensuremath{\approx}0.15$, with no indication of coexistence of SC and AFM in the vicinity of the QCP. Non-Fermi-liquid (NFL) behavior is observed in the normal-state electrical resistivity, $\ensuremath{\rho}(T)$, and specific heat, $C(T)$, in the vicinity of the QCP; e.g., the coefficient and the exponent of the power-law $T$ dependence of $\ensuremath{\rho}(T)$ exhibit pronounced maxima and minima, respectively, at ${x}_{\mathrm{QCP}}$, while $C(T)/T$ exhibits a logarithmic divergence in $T$ at ${x}_{\mathrm{QCP}}$. A low-temperature upturn in $\ensuremath{\rho}(T)$ develops in the range $0.70\ensuremath{\le}x\ensuremath{\le}0.85$ which is reminiscent of a single impurity Kondo effect, suggesting that Sm substitution tunes the relative strength of competing Kondo and Ruderman-Kittel-Kasuya-Yosida energy scales. The suppression of SC with increasing $x$ is probably associated with the exchange interaction between the Ce quasiparticles involved in the superconductivity and the magnetic moments of the Sm ions.

Short-range antiferromagnetic correlations in the superconducting state of filled skutterudite alloys Pr 1 -x Eu x Pt 4 Ge 12

Abstract: Motivated by current research efforts toward exploring the interplay between magnetism and superconductivity in multiband electronic systems, we have investigated the effects of Eu substitution through thermodynamic measurements on the superconducting filled skutterudite alloys ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}{\mathrm{Pt}}_{4}{\mathrm{Ge}}_{12}$. An increase in Eu concentration leads to a suppression of the superconducting transition temperature consistent with an increase of magnetic entropy due to Eu local moments. While the low-temperature heat capacity anomaly is present over the whole doping range, we find that in alloys with $x\ensuremath{\le}0.5$, the Schottky peaks in the heat capacity in the superconducting state appear to be due to Zeeman splitting by an internal magnetic field. Our theoretical modeling suggests that this field is a result of the short-range antiferromagnetic correlations between the europium ions. For the samples with $xg0.5$, the peaks in the heat capacity signal the onset of antiferromagnetic ordering of the Eu moments.

Tuning the magnetic ground state of Ce 1 − x Yb x RhIn 5 by Yb valence fluctuations

Abstract: We characterize the properties of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Yb}}_{x}{\mathrm{RhIn}}_{5}$ single crystals with $0\ensuremath{\le}x\ensuremath{\le}1$ using measurements of powder x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, magnetic susceptibility, specific heat, x-ray absorption near edge structure (XANES), and neutron diffraction. The Yb valence ${v}_{\text{Yb}}$, calculated from the magnetic susceptibility and measured using XANES, decreases from $3+$ at $x=0$ to $\ensuremath{\sim}2.1+$ at ${x}_{\mathrm{act}}=0.2$, where ${x}_{\mathrm{act}}$ is the measured Yb concentration. A transition from incommensurate to commensurate antiferromagnetism is observed in neutron diffraction measurements along $Q=(0.5,0.5,l)$ between $0.2\ensuremath{\le}{x}_{\mathrm{act}}\ensuremath{\le}0.27$; this narrative is supported by specific-heat measurements in which a second robust feature appears at a temperature ${T}_{I}\phantom{\rule{4pt}{0ex}}({T}_{I}l{T}_{N})$ for the same concentration range. Magnetic susceptibility measurements also reveal features which provide additional evidence of magnetic ordering. The results of this study suggest that the evolution of the Yb valence plays a critical role in tuning the magnetic ground state of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Yb}}_{x}{\mathrm{RhIn}}_{5}$.

The effective increase in atomic scale disorder by doping and superconductivity in Ca$_3$Rh$_4$Sn$_{13}$

Abstract: The comprehensive research of the electronic structure, thermodynamic and electrical transport properties reveals the existence of inhomogeneous superconductivity due to structural disorder in Ca$_3$Rh$_4$Sn$_{13}$ doped with La (Ca$_{3-x}$La$_x$Rh$_4$Sn$_{13}$) or Ce (Ca$_{3-x}$Ce$_x$Rh$_4$Sn$_{13}$) with superconducting critical temperatures $T_c^{\star}$ higher than those ($T_c$) observed in the parent compounds The $T-x$ diagrams and the entropy $S(x)_T$ isotherms well document the relation between degree of an atomic disorder and separation of the {\it high-temperature} $T_c^{\star}$ and $T_c$-bulk phases In these dirty superconductors with the mean free path much smaller than the coherence length, the Werthamer-Helfand-Hohenber theoretical model does not well fits the $H_{c2}(T)$ data We suggest that this can result from two-band superconductivity or from the presence of strong inhomogeneity in these systems The multiband model very well describes the $H-T$ dependencies, but the present results as well as our previous studies give arguments for the scenario based on the presence of nanoscopic inhomogeneity of the superconducting state We also revisited the nature of structural phase transition at $T^{\star}\sim 130-170$ K and documented that there might be another precursor transition at higher temperatures The impact of the magnetic Ce-Ce correlations on the increase of $T_c$ in respect to the critical temperatures of Ca$_{3-x}$La$_x$Rh$_4$Sn$_{13}$ is also discussed

Zero-field quantum critical point in Ce 0.91 Yb 0.09 CoIn 5

Abstract: We present results of specific heat, electrical resistance, and magnetoresistivity measurements on single crystals of the heavy-fermion superconducting alloy ${\mathrm{Ce}}_{0.91}{\mathrm{Yb}}_{0.09}{\mathrm{CoIn}}_{5}$. Non-Fermi-liquid to Fermi-liquid crossovers are clearly observed in the temperature dependence of the Sommerfeld coefficient $\ensuremath{\gamma}$ and resistivity data. Furthermore, we show that the Yb-doped sample with $x=0.09$ exhibits universality due to an underlying quantum phase transition without an applied magnetic field by utilizing the scaling analysis of $\ensuremath{\gamma}$. Fitting of the heat capacity and resistivity data based on existing theoretical models indicates that the zero-field quantum critical point is of antiferromagnetic origin. Finally, we found that at zero magnetic field the system undergoes a third-order phase transition at the temperature ${T}_{c3}\ensuremath{\approx}7$ K.

Observation of transition from semiconducting to metallic ground state in high-quality single crystalline FeSi

Abstract: We report anomalous physical properties of single-crystalline FeSi over a wide temperature range 1.8-400 K. X-ray diffraction, specific heat, and magnetization measurements indicate that the FeSi crystals synthesized in this study are of high quality with a very low concentration of magnetic impurities ($\sim$0.01$\%$). The electrical resistivity $\rho$($T$) can be described by activated behavior with an energy gap $\Delta$ = 57 meV between 67 K and 150 K. At temperatures below 67 K, $\rho$($T$) is significantly lower than an extrapolation of the activated behavior, and the Hall coefficient and magneto-resistivity undergo a sign change in this region. At $\sim$19 K, a transition from semiconducting to metallic-like behavior is observed with deceasing temperature. Whereas the transition temperature is very robust in a magnetic field, the magnitude of the resistivity below $\sim$30 K is very sensitive to magnetic field. There is no indication of a bulk phase transition or onset of magnetic order in the vicinity of either 67 K or 19 K from specific heat and magnetic susceptibility measurements. These measurements provide evidence for a conducting surface state in FeSi at low temperatures.